Home Original page

std::partial_order - cppreference.com

From cppreference.com

Defined in header <compare>

inline namespace /* unspecified */ { inline constexpr /* unspecified */ partial_order = /* unspecified */; }

 (since C++20)

Call signature

template< class T, class U > requires /* see below */ constexpr std::partial_ordering partial_order( T&& t, U&& u ) noexcept(/* see below */);

Compares two values using 3-way comparison and produces a result of type std::partial_ordering.

Let t and u be expressions and T and U denote decltype((t)) and decltype((u)) respectively, std::partial_order(t, u) is expression-equivalent to:

  • If std::is_same_v<std::decay_t<T>, std::decay_t<U>> is true:
    • std::partial_ordering(partial_order(t, u)), if it is a well-formed expression with overload resolution performed in a context that does not include a declaration of std::partial_order,
    • otherwise, std::partial_ordering(std::compare_three_way()(t, u)), if it is well-formed,
    • otherwise, std::partial_ordering(std::weak_order(t, u)), if it is well-formed.
  • In all other cases, the expression is ill-formed, which can result in substitution failure when it appears in the immediate context of a template instantiation.

Customization point objects

The name std::partial_order denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.

Example

See also

the result type of 3-way comparison that supports all 6 operators, is not substitutable, and allows incomparable values
(class) [edit]
performs 3-way comparison and produces a result of type std::strong_ordering
(customization point object)[edit]
performs 3-way comparison and produces a result of type std::weak_ordering
(customization point object)[edit]
performs 3-way comparison and produces a result of type std::partial_ordering, even if operator<=> is unavailable
(customization point object)[edit]